Recently, the International Organization for Standardization (ISO) released an international standard in the field of microbeam analysis: Microbeam analysis — Scanning electron microscopy — Method for evaluating critical dimensions by CD-SEM (ISO 21466). The standard was developed by a team led by Prof. DING Zejun from the Department of Physics, University of Science and Technology of China (USTC) and the National Laboratory for Physical Sciences at the Microscale. As the first ISO for the linewidth of semiconductors and the first ISO developed by China in semiconductor, the standard not only promotes the semiconductor evaluation technology but also enhances the China's international influence and competitiveness in the semiconductor industry.
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In the semiconductor industry, it is more and more precise controlling the size of integrated circuit devices. The physical size features on a chip are called feature sizes, while the smallest feature size is called the critical dimension (CD) which represents the complexity level of the semiconductor manufacturing. The measurement for CD is called nanometer-scale linewidth measurement. At present, the etching linewidth of semiconductors has fallen below 10 nm. The accuracy of the measurement directly determines the performance of the device. The accurate and precise measurement of nano-device scale (accuracy <1 nm) plays a vital role in the development of the whole semiconductor industry, which is indeed a very challenging task. Various measurement techniques have been applied, such as scattering measurement, atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Critical dimension scanning electron microscope (CD-SEM) is the easiest and most efficient way to perform real-time monitoring and linewidth measurement in the semiconductor industry. However, the SEM has the enhanced effect of the secondary electron signal emission at the edge of linewidth. The analysis of CD-SEM images at nanoscale requires a high-precision algorithm.
Prof. DING Zejun and his colleagues have long been engaged in the fundamental researches on the interaction of electron beam and materials. They invented the most advanced Monte Carlo simulation calculation method for SEM and surface electron spectroscopy over the world. These researchers proposed an ISO method “evaluating critical dimensions by CD-SEM” based on a Model-based Library (MBL) approach from NIST. By imaging with CD-SEM, this document specifies the structure models and parameters of linewidth, Monte Carlo simulation models, imaging scan line calculations, the method and file format of MBL, image matching and fitting program, and CD parameter setting. Compared with the traditional threshold method, this standard can give an accurate CD value, and extend the linewidth measurement from evaluating a single parameter to including the topographic features, which is appropriate for gate on wafer, photomask, single isolated or dense line feature pattern down to size of 10 nm. The standard sets the first industry standard for semiconductor linewidth determination by CD-SEM. Meanwhile, it provides a reference for other general nanoscale measurements.
Since 2011, the team had worked on the research SEM-based nanoscale measurement model funded by 973 Project. They made a report on the new standard proposal in ISO/TC202/SC4, 2014 and submitted the proposal for voting in 2015. And then, the proposal was approved to set up a project formally by the International Standard (IS) in May 2016. The draft standard began to undergo four rounds of voting by member states. Finally, the standard was passed on 27 September 2019 and has been officially published now (https://www.iso.org/standard/70944.html).
The main members involved in the research and development of the standard are as follows: Dr. ZHOU Yanbo (Xinjiang Normal University), Dr. LI Yonggang (Hefei National Laboratory for Physical Sciences at the Microscale), and Dr. LI Huimin (USTC). This research was supported by 973 Project of Ministry of Science and Technology, National Natural Science Foundation of China, and Supercomputing Center of USTC.
Reference
1. https://www.iso.org/standard/70944.html
(Written by ZHAO Xiaona, edited by JIANG Pengcen, USTC News Center)